Are you getting any kind of error message? I remember in the earlier releases of the 200 ICP, the Telnet would lock up requiring a reboot. They fixed that though. Are you sure you are using the correct username and password? Have you tried a lower level username and password? I would check that first. If the system is not showing you any love, then I would reboot the system and see if you can get in. If you cannot gain access after a reboot, then I am pretty sure you have a password problem. Maybe someone else has been in there?
If you determine that you do have a changed password, but don't know what it is, Mitel can reset it for a fee!
Good Luck
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My Biggest problem is that I almost always believe what I tell myself.